Radio Mast - motor does not stop

Hi All, 2 days to Xmas !!
 I've been searching the web and can't find this problem/solution. Only problems like mast won't go up or down.
 SW20 came with the mast missing and the antenna motor not making a sound. Junior worked out why the antenna motor did not work. One or both of the up/down signal wires had a weak connection which he fixed. Then he installed the new mast as per instructions on the web and used the motor (by turning the radio off) to retract the aerial fully. Yay ! Now time to test it going up and the problem is that when it goes to the full height and stops, the motor keeps going and some associated noise comes from inside the gear drive unit. So it seems the limit detection for UP is not working. Does anyone know how that works and where to look to diagnose ?

OK well it seems the spoiler sleeve is supposed to come with the aerial so numbnut (me) bought the wrong aerial type  (they have another type for sw20 that don't go through the spoiler) !
hmmmmm...............I should have a glass of red....

Thanks Matt, yep the piece in question which was on the car so not removed with the previous aerial,
is a part of the replacement antenna. Unfortunately I had already bought the other type of SW20 antenna which does not go up through the spoiler. I am pretty sure that everything will work properly if I get the correct aerial which incorporates the spoiler sleeve.